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52 DESIGN007 MAGAZINE I JUNE 2023 As a result, designers and fabricators are looking for newer-generation surface finishes to meet their performance criteria. EPIG (electroless palladium immersion gold with no EN), silver-gold (Ag-Au), and reducing the EN thickness from traditional ENEPIG, have all gained attention. What follows is a review that compares the performance attributes of the leading candidates for a high-frequency alternative surface finish. MacDermid Alpha Electronic Solutions has worked in partnership with Rogers Corpora- tion to evaluate the effect of various surface fin- ishes on signal loss with increasing frequency. Together, we subsequently worked to under- stand and compare other critical-to-quality performance metrics that will guide applica- tion-based surface finish selection. Work Program We first set out to evaluate the effect of sur- face finish on signal losses up to 110 GHz using the microstrip test method and a vector net- work analyzer, with the support of Rogers. e test candidate surface finishes selected are: • Standard ENEPIG (4 µm electroless nickel/0.1µm palladium/0.05 µm gold) • Standard ENIG (4 µm electroless nickel/0.05 µm gold) • in EN ENEPIG (0.2 µm electroless nickel/0.1 µm palladium/0.05 µm gold) • Ultra-in EN ENEPIG (0.1 µm electroless nickel/0.1 µm palladium/ 0.05 µm gold) • EPIG (0.1 µm palladium/0.05 µm gold) • Ag-Au (0.15 µm silver/0.05 µm gold) • Immersion Silver (0.3 µm) • OSP (0.4 µm) Following the evaluation of insertion losses, each surface finish was assessed based on other performance criteria: • High-speed ball shear • Solder spread testing • Drop shock evaluation • Gold and aluminum wire bonding • Solder joint electromigration Aer data collection, it was summarized, and a decision matrix was constructed to allow designers to compare performance require- ments against each surface finish's capabilities. Insertion Loss Testing e total insertion loss through a microstrip circuitry, α T , comprises four different loss com- ponents as shown in Equation 1: where α D , α C , α R , and α L represent the dielec- tric loss, the conductor loss, the radiation loss, and the leakage loss, respectively. If the trans- mission line is an ideal impedance-match- ing circuit, the total transmission loss can be expressed using only two dominant factors, i.e., the conductor and dielectric losses, as shown in Equation 2 1 . Multiple factors affect the conductor loss of the transmission line, including surface rough- ness of the copper foil, skin effect, and mag- netic permeability of the conductor. e skin effect is closely related to the surface finishes. When the frequency of the signal increases, the current flowing in the transmission line focuses on the surface of the copper foil, instead of the center of the foil. is is known as the skin effect. e skin depth δ amplitude of the cur- rent flowing on the surface of the transmission line can be derived using Equation 3 below 2,3 .